The identification of the arrangement of interior features of solid objects using Computed Tomography (CT) is a well-established field, with typical applications in medicine and industrial quality control inspection. Commonly, the object of interest has a general shape, so the most general form of CT analysis must be used, with consequent requirements for very sophisticated equipment, extensive measurements and substantial mathematical calculations. In the case of objects with known patterns of overall shape and internal features (“a-priori knowledge”), significant savings can be made in equipment sophistication, number of needed measurements and size of mathematical calculation by building the a-priori knowledge into the calculation.
Log quality assessment is an important need in wood processing operations to enable informed choices to be made for subsequent log processing. A great benefit can be obtained by knowing in advance the most advantageous purpose to which each log can be put, for example, for the production of veneer, specialty woods, dimension lumber or pulp chips. In general, the best economic result is achieved by matching each log to the highest value application to which it is suited. Conversely, a great benefit can also be obtained by knowing in advance to what purposes each log cannot be put. For example, low quality logs that cannot produce useful sawn products should be diverted away from entering a sawmill, thus avoiding much redundant and costly material handling.
Log quality assessment has traditionally been done visually by skilled workers. The shape of the outside surface gives important clues to log characteristics, allowing a first rough sort of the cut logs to be made in the forest at the time of cutting. Further, mechanized inspection at the sawmill, commonly based on optical measurement of the surface shape of the logs, provides a more detailed assessment. However, it is often challenging to try to infer the interior features of a log based on surface shape measurements. Many interior features cannot be easily identified by examination of the surface, and thus go undetected.
X-ray inspection of has been introduced as a means of viewing the interior of logs. Typical systems involve making radiographs of each log, thus producing images of a log analogous to a chest X-ray image of a person. In U.S. Pat. No. 7,149,633, Woods et al. describe a procedure for inspecting sawn boards from radiographs. Such radiographs can provide much useful information about each log, but they are limited to providing 2-dimensional results. They cannot indicate the 3-dimensional character of the interior structure.
X-ray inspection from two or more directions has been introduced to provide the missing information in the third dimension. Aune and So describe such a system in U.S. Pat. No. 5,023,805, also Poon in U.S. Pat. No. 5,394,342, and Skatter in U.S. Pat. No. 6,757,354. Such systems have been only partially successful because it is very difficult to infer 3-dimensional information from radiographic measurements in a small number of directions. Typically, measurements need to be made in a large number of directions to allow 3-dimensional information to be inferred with confidence.
In an effort to enhance the capabilities of multi-directional imaging, medical style Computed Tomography (CT) has introduced for sawmill use. Schmoldt et al. summarize some typical applications in “Nondestructive Evaluation of Hardwood Logs: CT Scanning, Machine Vision and Data Utilization”, published in Nondestructive Testing and Evaluation, Vol. 15, pp. 279-309, 1999. The technique involves making high-resolution X-ray measurements in a very large number of directions, possibly exceeding 1000. A typical arrangement is to rotate an X-ray source and opposing detector array around the specimen, making measurements at numerous angular steps around the rotation path. With conventional single-slice systems, all measurements during a given rotation are contained within a single cross-section. The X-ray detectors are set along a line within the plane of the cross-section. When all measurements in one cross-sectional slice are completed, the system moves on to measure the next cross-sectional slice, and so on for each slice individually.
Single-slice CT systems tend to be relatively slow because they measure only one slice at a time. In addition, they make very inefficient use of the X-ray beam because they use only a small part of it along a narrow line. As described by Seger and Danielsson in “Scanning of logs with linear cone-beam Tomography”, published in Computers and Electronics in Agriculture, Vol. 41, pp. 45-62, 2003, greater use of the X-ray beam can be made by installing additional X-ray detectors along rows parallel to the central row. They provide additional measurements that can improve the stability of the resulting reconstructions.
The need to make measurements in separate steps at discrete cross-sections along the length of the measured object makes the slice-by-slice style of CT measurement inconvenient for industrial use with logs. An alternative approach is to make measurements while the X-ray source and detector array follow a continuous spiral path around the specimen. Such systems are now well developed for medical applications, as described by Kalender et al. in “Spiral CT Medical Use and Potential Industrial Applications”, published in SPIE Vol. 3149, pp. 188-202, 1997. Garms describes an industrial application in U.S. Pat. No. 6,778,681.
CT measurements require that the relative motions of the scanner and the measured object are very precise and well defined, else artefacts are created in the CT reconstruction. Such accurate relative motions are achieved in medical scanners by rotating the X-ray source and detectors within large mechanical bearings while advancing the patient along a precise linear path. This is a very complex and costly arrangement. Several approaches have been developed to reduce reconstruction artefacts, for example, as described by Edic in U.S. Pat. No. 7,382,852, and Weese in U.S. Pat. No. 7,558,439, but accurate relative motions are still needed.
In U.S. Pat. No. 6,157,698, Pietikainen and Alisto describe the use sector-shaped voxels with annular boundaries indicated at equal radial intervals. This use of voxels with greatly dissimilar volumes gives poor results for the small interior voxels. The disclosed procedure uses planar cross-sections, and thus extensive post-processing of the reconstruction results is required to identify knots. In addition, the focus is on knot identification, without consideration of the use of annular voxels without sector division as a means for identifying axisymmetric features.
In U.S. Pat. No. 6,597,761, Garms describes the use of cylindrical projections for log evaluation. This process provides a post-processing step to assist interpretation of the results of a conventional CT measurement using many small rectangular voxels. Thus, all the requirements of conventional CT measurements must still be met, for example, many fine-resolution measurements, maintenance of very accurate relative motions, and very large computational effort. By defining the voxel arrangement as described herein, all these requirements may be relaxed significantly, and a much more efficient and economical CT measurement can be achieved.
Even with all these developments, it remains very challenging to try to implement CT methodology for practical industrial use. Such applications cannot tolerate the high cost, complexity and modest speed that are acceptable in medical systems. What is required is an industrial system of moderate cost and complexity, and of sufficient speed to make measurements in “real-time”, so that it can keep up with product flow without causing delay. Therefore, there is a need for achieving these objectives by making and using the measured X-ray data in a much more effective way.